to_pops_barplot: make a barplot that has different densities of the same color...
In eriqande/herring: genetic stock identification of river herring

make a barplot that has different densities of the same color for pops within the same reporting unit

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to_pops_barplot(ass_array, rep_groups,
  col.pairs = matrix(c(brewerPairs("Reds"), brewerPairs("Blues"),
  brewerPairs("Greens"), "yellow", "khaki"), nrow = 2))

`ass_array`	An assignment array with rows being from and cols being "to". Where individuals are assigned to populations.
`rep_groups`	a factor vector given the reporting groups each populations belongs to. For this to work, the pops must all be ordered so that they are all next to one another within reporting groups
`col.pairs`	matrix of pairs of colors